This invention relates to a spiral wound filtration cartridge construction utilized in fluid separation processes such as reverse osmosis or ultrafiltration. More particularly, this invention relates to a spiral wound filtration cartridge construction having a partial feed bypass means which prevents fluid stagnation within and around the cartridge during use.
Prior to the present invention, spiral wound filtration cartridges have been provided comprising a centrally positioned tube about which are wound membrane sheets and suitable permeate spacers and feed spacers. The membrane sheets are V shaped and the feed spacers are positioned within the shaped membrane while the permeate spacers are positioned outside of the V shaped membranes. The feed spacers promote passage of feed fluid into the V shaped membrane and the permeate spacers promote permeate flow from outside the V-shaped membrane into holes through the walls of the centrally positioned conduit for recovery. The feed passes axially through the spiral wound membrane cartridge and retentate is recovered from the end of the cartridge opposite the end at which the feed is introduced. The filtration cartridge is held within a cylindrically shaped housing to provide mechanical strength to the cartridge during use.
It has been proposed to provide a controlled by pass of feed fluid within the space between the housing and the outside surface of the cartridge in order to prevent bacterial growth during use caused by clogging of the cartridge with retentate. U.S. Pat. No. 4,301,013 discloses the use of a tight fitting open mesh within the space between the cartridge and the housing to provide the desired by-pass flow. It has also been proposed in U.S Pat. No. 4,548,714 to wrap the cartridge with a leaf of the feed spacer to provide for fluid flow around the cartridge. It has also been proposed in U.S. Pat. No. 4,906,372 to utilize a seamless porous rigid sleeve within the housing to separate the cartridge from the housing and to provide for feed bypass. These cartridges are difficult to produce with the requisite tolerance needed to permit their use in the field, particularly when it is desired to change the cartridge within the housing. That is, these cartridge and housing arrangements are difficult to replace in the field since each cartridge-housing arrangement varies in size and shape.
It has also been proposed to render the cartridge more uniform in size by wrapping them with an adhesive tape as disclosed in U.S. Pat. No. 3,966,616 or with wound glass filament bonded with epoxy or by heat shrinking a sleeve about the cartridge. The use of tape or bonded filaments is undesirable since the tape seam or filaments retain solids in the feed thereby causing dead space or stagnation areas and resultant bacterial growth. The use of a heat shrink wrapping also is undesirable since the cartridges formed therewith are inconsistent in size and shape, have limited strength and the wrap has a tendency to slip off. It has also been proposed to surround the cartridge with a flexible sealing gasket to effect a seal between the outer housing and the cartridge. Small holes are provided through the flexible gasket to effect fluid flow between the outer housing and the cartridge. This arrangement is undesirable because the holes become deformed or blocked during use under high pressure.
Accordingly, it would be desirable to provide a filtration cartridge construction which affords a controlled feed by-pass means to maintain the construction clean during use. It also would be desirable to provide a filtration cartridge construction which can be easily utilized in the field to permit cartridge replacement when needed. It would also be desirable to provide a construction wherein the by pass of feed fluid can be controlled within narrow tolerances.